The invention relates to a self-energizing disc brake having an electric actuator in which an application force is amplified using a self-energizing device, and to a method for activating a self-energizing brake.
Self-energizing brakes are known in a wide variety of types. For example a classic design of self-energizing brakes are drum brakes in which a brake shoe is arranged in a leading fashion so that the friction forces between the brake lining and drum support the tensioning forces.
In contrast, in disc brakes it was assumed in the past that it was in fact a significant advantage of this design of brake that, with brake linings which act exclusively perpendicularly to the circumferential brake disc and on which only an activation device which acts parallel to the axis of the brake disc acts with a force which is aligned in such a way, there is no self-energizing effect. This was the case to an even greater extent in disc brakes for heavy utility vehicles in which the activation is preferably carried out hydraulically or pneumatically.
However, if disc brakes with activation devices which are operated electromotively are also to be used in relatively heavy commercial vehicles, the self-energizing disc brake becomes an option since it provides the possibility that, owing to the self-energization of the brake, the electric motor can be given smaller dimensions than would be possible with a non-self-energizing disc brake.
Self-energizing disc brakes are known in a wide variety of designs. However, the majority of solutions describe operational principles which permit self-energization but, due to a lack of price competitiveness as well as due to their awkward complex design, are not suitable for implementing a disc brake for heavy commercial vehicles which is ready for series production and can be manufactured economically, and often therefore have not passed the stage of theoretical ideas.
Against this background, the present invention provides an electromechanically operated, self-energizing disc brake which can be manufactured cost-effectively with a simple design. It preferably also provides the advantage that the power demand of the electromotive drive is minimized compared to comparable, directly electromechanically activated brakes by using efficient self-energization, even in the boundary region of the coefficient of friction of the brake lining.
The invention achieves this result using a brake application unit. Having two or more brake plungers, each plunger having pressure surfaces with a recess and ramp shaped contours into which engage rolling elements.
Accordingly, each of the pressure surfaces of the at least two or more brake plungers is provided, on its side facing the brake lining unit, with a recess with a ramp shaped contour into each of which a rolling element engages which is supported at one end on the ramp shaped contour of the pressure surfaces of the brake plungers and at the other end on the brake lining unit.
In this way, the self-energizing brake is easily combined with an adjustment device. The brake is applied in the direction of the brake disc or perpendicularly to the brake disc by the brake plungers by changing their length in the axial direction. The application of the brake in the circumferential direction or parallel to the disc is carried out in a different way, for example with a crank. The brake plungers can also be used to adjust for brake lining wear. For braking operations this advantageously results in a very wide variety of options which will be explained in more detail below.
The invention preferably also implements a configuration of the brake application unit such that it converts uniform rotations of an output shaft of the electromotive drive during a brake application movement into a movement of the brake lining, the movement component of which movement is nonlinear at least in the tangential direction (direction U).
Advantageous embodiments of the invention will be described below.
The invention not only reduces the manufacturing costs of a brake system for utility vehicles but it also significantly minimizes the power demand of the electromotive drive in relation to comparable directly electromechanically activated brakes by using efficient self-energization even in the boundary region of the coefficient of friction of the brake lining. According to particularly advantageous embodiments, it is possible to significantly reduce the power demand compared to other known self-energizing concepts.
It is also possible to meet the same power requirements compared to modern compressed air brakes and also to satisfy the same predefined installation conditions and weight prescriptions.
The adjustable ramp system can also be used to implement a reliable parking brake which adjusts automatically even when friction elements shrink owing to cooling. A further significant advantage of the invention is therefore the fact that with the proposed disc brake a reliably acting parking brake is also implemented without additionally necessary activation components.
For this purpose, the ramp angle with the greatest degree of self-energization must be dimensioned in such a way that self-energization is possible even at the lowest envisioned coefficient of friction of the brake lining.
When the brake is inserted, there is therefore an exclusively mechanically holding effect of the brake. If brake linings and/or the brake disc shrink or if there is a drop in the coefficient of friction which occurs during the shut-off phase, the brake and the self-energization of the brake are automatically adjusted in order to keep the vehicle in a stationary state.
The electromotive drive is preferably coupled to an open-loop and/or closed-loop control device which is configured to perform respectively open-loop or closed-loop control of the position of the actuator element or brake lining. In this context, the position of the brake lining unit is set according to predefined values of a superordinate unit (for example a control device).
This open- and/or closed-loop control device is preferably operated as follows:
The basis of the preferred closed-loop control concept is a braking or deceleration closed-loop control process of the vehicle such as is customary in contemporary EBS closed-loop-controlled vehicles with a compressed air brake system.
In such brake systems, the driver or an autonomous vehicle system presets a braking request or deceleration request which is converted into a “braking” signal which is processed by the EBS system and converted into a corresponding actuation of the wheel brake actuators (pneumatic cylinders or electric motor) which brings about corresponding activation of the brakes.
In pneumatically activated disc brakes, a pure pressure control process of the activation cylinder of the respective brake is usually carried out according to the relationship:                brake pressure→cylinder force→tensioning force frictional force which can be determined and reproduced within sufficiently tight limits of precision.        
In self-energizing, electromagnetically activated brakes, this sufficient precision is generally no longer provided between the actuator manipulated variable and the frictional force.
The motor current is frequently used as the activator manipulated variable of electromechanically self-energizing brakes of this kind. However, such large tolerances of the achievable braking effect arise from the engine efficiency levels, which are for example also temperature dependent, and from the efficiency level of the step down gear mechanism as well as finally the efficiency level of the amplification method in conjunction with the variations in the coefficient of friction of the brake linings, that it no longer appears possible to control the braking effect by using the motor current.
It has been proposed to measure the frictional force and carry out closed-loop control on it directly for example in International patent document WO 03/100282 describing the self-energizing brake which has wedge activation and is described in European patent document EP 0 953 785 B1.
According to this method there is the problem of finding a suitable measuring method for determining the frictional force. Furthermore, there is the difficulty of the frictional source being influenced to a very high degree by brake oscillations and wheel oscillations and thus constituting a controlled variable which can be controlled only with difficulty.
The aim is therefore to find a closed-loop control method for self-energizing brakes which is well suited in particular also to disc brakes and which avoids the problems associated with closed-loop control of the frictional force.
To summarize, the present invention implements a method for actuating a self-energizing brake in which an activation force applied by the actuator is amplified using a self-energizing device arranged between the actuator and brake lining, wherein the actuator is coupled to an open-loop or closed-loop control device which is configured to actuate the actuator in order to set the position of the brake lining units in such a way. The present method is distinguished by the fact that during the closed-loop control process tolerance-conditioned braking force differences, referred to as third controlled variable, among the wheel brakes on which the closed-loop control process is performed by the brake system, are determined and compensated.
The invention also provides a method for carrying out a parking braking operation, with a brake according to the invention in which during a parking braking operation the brake is applied in a simple manner solely using the brake plungers until the rolling elements have moved the lining units against the disc, after which the self-energizing effect starts without the crank being activated.
The brake plungers or pistons can also be used for relatively minor compensation braking operations.
As far as possible, sensor systems which are already present and are reliable and proven are used to sense the signals which are necessary for the closed-loop control.
A First Variant Will be Explained First
Solution 1: Third Controlled Variable
The solution described below provides a brake system in which, between the “braking or deceleration” vehicle controlled variable and the “current or actuator position” actuator manipulated variable, a third controlled variable is introduced which is intended to compensate the tolerance-conditioned braking force differences among the wheel brakes on which closed-loop control is carried out by the brake system.
This third controlled variable is sensed individually for each vehicle wheel and compared with the values determined at the other wheels.
When there are inadmissible deviations from the defined values of the EBS system, these predefined values (motor current or actuator position) for the individual brakes have a correction factor superimposed on them individually, with which correction factor the existing braking force differences are compensated.
This adaptation process is carried out, if appropriate, in relatively small increments over a plurality of brake activation processes.
The wheel slip of the respective vehicle wheel is preferably evaluated as a third controlled variable.
In this method it is advantageously unnecessary to generate a precise relationship between the wheel slip and braking force, but rather the wheel slip characteristic variables which occur at the individual wheels are adjusted to form a specific predefined set point value for the EBS system. In particular, in this context the wheel slip characteristic variables of the brakes of the individual axles are adjusted as precisely as possible. The matching of the wheel slip characteristic variables of the axles to one another takes place in a second step taking into account the possibly different predefined values of the brake system for the individual axles.
Alternatively, the tensioning force acting on the brake can also be determined as the third controlled variable. The tensioning force can be determined at the components of the brake which pick up force, for example at the brake caliper, by measuring deformation paths or component stresses. In the process, the necessary sensor can be arranged in the interior of the brake and integrated, if appropriate, into an electronic control system which is arranged within the brake.
Solution 2: Open Loop Control Using Actuator Position or Motor Current Combined with Tolerance Compensation
A second approach to the solution is based on the existing control algorithm of contemporary EBS systems in which only the actuator manipulated variable of pressure is replaced by another system-specific manipulated variable. The actuator position and motor current are particularly appropriate as system-specific manipulated variables.
In the description of the prior art, the large tolerance variation, which makes this method more difficult to apply, has already been mentioned. It is therefore necessary to largely eliminate the tolerance influences present in this effect chain.
This is preferably brought about with one or more of the following measures:                Before the brake actuator is activated, the venting play is overcome by using the adjustment device so that the venting play is already no longer present as a fault source at the start of the actual brake application movement by the brake actuator.        The influence of the brake lining compression—which differs due to the wear state and temperature state—on the predefined set point values of the brake system is compensated by correction factors. For this purpose, the wear state of the two brake linings is determined precisely for each brake. Likewise, by evaluating the energy balance of the brakes, their thermal content and hence also the temperature of the brake lining are determined. This energy balance can be evaluated by the electronic brake system or by an electronic open-loop controller which is integrated into the brakes.        Brake-specific variations in the relationship between the tensioning force and the widening of the caliper are compensated by a calibration process when the brakes are fitted. For this purpose, defined forces are applied to the brake caliper, for example during the final inspection on the assembly belt, and the widening which occurs in the process is determined or the actuator adjustment travel necessary for this is determined directly. The defined application of force is preferably carried out in such a way that force pickups are used in the brake caliper, for example instead of the brake disc, and the actuator is then actuated in order to generate the predefined tensioning forces. The relationship between the tensioning force and the actuator position which is detected in this way can then be stored, for example in an electronic system which is integrated into the brakes.        When the motor current is applied as an actuator variable, the tolerance compensation which is described can be applied in the same way. The relationship between the tensioning force and the motor current is then determined during the calibration process and stored as described above. During this calibration process, the tolerance influences of the gear mechanism and electric motor are also eliminated at least for room temperature conditions. The temperature influence on the electric motor, for example on its permanent magnet, can in turn be compensated by the abovementioned thermal balance calculation.        
The resulting normal force for a specific position of the self-energizing device is dependent on a large number of factors such as    current venting play    rigidity of the brake (caliper) perpendicular to the frictional surface    in particular the variable rigidity of the lining which is dependent on
locations of linings
wear state, that is to say residual thickness
temperature
prior history (effect on compressibility)
take up of moisture                variable temperature of caliper and disc during the braking process        coefficient of friction between the brake lining and brake disc (effect on self-energizing effect and thus also on the normal force and on the frictional force). This is itself dependent, inter alia, on        
temperature
speed
According to the teaching of the invention, selective actuation of the ramp position in order to bring about a specific pressing force is virtually impossible if the influence of the aforesaid parameters is disregarded entirely.
In contrast, by virtue of the invention, a desired brake lining pressing force can be brought about by selective travel control of the self-energizing device or of the brake lining, and it is thus possible to dispense with a difficult-to-implement adjustment of the setpoint value to the actual value of the frictional force or else to permit selective pilot control for a brake with a setpoint value/actual value comparison of the brake lining pressing force or other frictional force.
According to the invention, this is achieved by virtue of the fact that interference variables which influence the correlation between the ramp position or brake lining position and the brake lining pressing force are compensated by taking into account relevant parameters.
For this purpose, a characteristic curve is determined which defines a corresponding pressing force in accordance with a position of the self-energizing device, for example a ramp, or an actuation travel which is predefined by the actuator.
This characteristic curve is preferably updated continuously, in order, for example, to be able to take into account influences such as temperature and speed.
The application point of the brake lining on the brake disc is determined, for example using the current of an electric actuator or by calculating it from the current venting play and ramp geometry.
The positive gradient of the characteristic curve is adapted as a function of the ramp position or brake lining position to:    a) Rigidity of the brake (caliper) perpendicular to the frictional surface can be determined experimentally or by calculation and is substantially constant.    b) In particular the variable rigidity of the lining which is dependent on            brake lining locations either by specification within a tolerable framework or by inputting/selecting corresponding parameters in an electronic control device when the brake lining is changed.        wear state, that is to say residual thickness is sensed continuously (as in the prior art)        temperature either by measurement or by calculation, for example using energy integration, cooling power etc.        prior history (such as effect on compressibility) logging of the prior history of the brake lining (aging), for example using energy integration, maximum temperature or the like. Relationship between the rigidity of the brake lining and aging can be determined empirically.            c) The temperature of the caliper and disc which varies during the braking process either by measurement (for example thermal elements) or calculation.    d) Coefficient of friction between the brake lining and brake disc (effect on the self-energizing effect and thus on the normal force and on the frictional force).     This is itself dependent, inter alia, on            temperature        speed        empirical determination of the dependents.        
Alternatively or additionally, closed-loop control of the brake can also be carried out by determining the normal force which acts between the brake lining and disc. The normal force can be determined, for example, by sensing the expansion of the calipers. If the actual normal force deviates from the desired normal force, the latter can be adapted by the described travel/force characteristic curve.
The brake application unit or ramp can be implemented in a defined fashion by an angle, either as a pressure ramp, traction ramp or traction/pressure ramp. In the case of a traction/pressure ramp in particular a self-locking system is advantageously selected as a drive, i.e. a high force which results in the direction of the activation from an unusually high/low coefficient of friction cannot lead to uncontrollable displacement of the ramp.
The described compensation of the interference variables can also be used for directly activated systems (activation force=support force).
As a further embodiment of the invention, there is provision for the electric motor to rotate a crank directly or by using at least one or more gear mechanisms, the crank having a crank tappet as output element which serves to move the brake lining unit, and the crank tappet is oriented parallel to the axis of the brake disc. The arrangement is compact and easy to implement in structural terms.
In this context, the electric motor preferably has an output shaft which is oriented parallel to the axis of the brake disc and by which the crank which acts on the brake lining unit is rotated directly or using further, intermediately connected gear mechanism elements.
According to a further variant of the invention, the brake application unit also has at least one, in particular two or more, brake plungers, used as adjustment for pistons, which are oriented parallel to the axis BA of the brake disc and which are supported at one of their ends on the brake caliper or by a bearing device on a component which is connected to the brake caliper, the bearing device permitting in each case at least some of the brake plungers to rotate about their longitudinal axis.
To summarize, the following advantages are provided, each independently and also in combination:    Circumferential activation by a crank
coaxially arranged drive unit
preferably integrated electronic control system    Simple combination of spindle actuation and/or crank activation application function by spindles—force stroke by means of crank application and adaptation braking using spindles crank activation for high-load braking operations parking brake function using spindles    Reliable and uncomplicated parking brake function    Pretensioning by spindles—automatic post-tensioning by the amplification system without crank activation.    If appropriate additional post-tensioning by crank activation    Addition of a highly amplifying ramp angle    Use of a common drive Shiftable distribution gearing    Automatically shifting (only application function with spindles)    Extraneously shifted (parking function and partial load braking function with spindles)    Variable self-energization    Multi-stage, shiftable infinitely variable, automatically adaptive and/or extraneously controlled    Controlled self-locking of the brake plungers    Controlled, and in the event of faults automatic, switching over from self-locking to non-self-locking operation    a) self-locking spindles and addition of a non-self-locking preliminary stage (folding ramp, spherical ramp etc.)    b) non-self-locking spindles and addition of a preliminary stage which brings about self locking (self-locking gear stage etc.)    Play-free drive
Measures for eliminating play in the force transmission path from the drive motor to the brake lining pressure plate.